Improve treatment of disabled symbols and custom symbols.

doc/src/engine/custom-syntax.md

@@ -68,11 +68,13 @@ These symbol types can be used:
 ### The First Symbol Must be a Keyword
 
 There is no specific limit on the combination and sequencing of each symbol type,
-except the _first_ symbol which must be a [custom keyword].
+except the _first_ symbol which must be a custom keyword that follows the naming rules
+of [variables].
 
-It _cannot_ be a [built-in keyword]({{rootUrl}}/appendix/keywords.md).
+The first symbol also cannot be a reserved [keyword], unless that keyword
+has been [disabled][disable keywords and operators].
 
-However, it _may_ be a built-in keyword that has been [disabled][disable keywords and operators].
+In other words, any valid identifier that is not an active [keyword] will work fine.
 
 ### The First Symbol Must be Unique
 

doc/src/engine/disable.md

@@ -20,8 +20,7 @@ engine
 // The following all return parse errors.
 
 engine.compile("let x = if true { 42 } else { 0 };")?;
-//                        ^ missing ';' after statement end
-//                      ^ 'if' is parsed as a variable name
+//                      ^ 'if' is rejected as a reserved keyword
 
 engine.compile("let x = 40 + 2; x += 1;")?;
 //                                ^ '+=' is not recognized as an operator